I've been keeping and breeding Testudo species for over 20 years, including Russians. I live in the Northeastern U.S. and keep mine outdoors year round. The only supplement that I have ever given them is calcium carbonate in the form of cuttlebone. Most of my Russians have been here for at least 16 years. They eat the cuttlebone as they desire and I have never had any issues with soft shells or other problems associated with vitamin D or lack of it. Some people love to complicate keeping tortoises. Keep it simple and you'll be much more successful at it.